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Flood Proclamation file

The Clark Fork River flows over a diversion dam near the Van Buren Street footbridge in Missoula recently. 

A new study slated for this spring will use aerial photography to get a better idea of where the Clark Fork and Bitterroot river channels are moving.

The $32,000 study comes one year after flooding pushed the Clark Fork out of its banks near the Orchard Homes and Tower Street areas, carving new channels that may continue to carry river water into the future.

“It’s an important and timely project,” said Travis Ross with the Missoula Water Quality Protection District. “The results will be presented publicly and hopefully they can be used to better inform planning in the future.”

Access Geographic, an Arizona-based geospatial mapping firm, will fly over Missoula County to take aerial photographs of the two rivers. The company's Rob Moore said they work about 15,000 feet in the air, using GPS to follow grid patterns so the still camera can create 3-D images.

"Basically, it's like mowing the lawn," Moore said. "We do overlapping lines in one direction, collect everything in stereo so we can have multiple perspectives of the terrain. That helps us create the 3-D orthophotography."

Moore added that his pilots are in constant contact with the flight tower in order to ensure everyone's safety.

Ross said he’s not sure when the flights will take place, since the window of opportunity is limited by water levels, weather and leaf densities.

“It could be done next week, or it could be done in June,” Ross said. “The conditions have to be right to fly. There can’t be any cloud cover. The wind speed is important, as are the weather considerations and the flooding. If the rivers start flooding again we don’t have an accurate picture of the active main channel. So you have to hit the sweet spot.”

Moore said Missoula is particularly challenging, with its tendency for cloud-free morning skies and afternoon storms.

"We take hundreds, sometimes thousands, of images over a period of time … and need a seamless element of continuity," Moore said. "We need every photo to be consistent so when they overlap to create the map they're consistent." 

This next week could be prime weather for taking aerial photographs, according to the forecast by Bob Nester with the National Weather Service. He expects only a few showers on Tuesday and Wednesday, followed by clear skies and temperatures into the 70s and 80s. He noted that the National Weather Service’s hydrologic gauge on the Clark Fork River above Missoula shows flows will begin trending upward on Saturday, but remain below flood stage — about 6.7 feet —through Tuesday.

“We’re building a ridge of high pressure later this week, and might even see 80 degrees in Missoula this weekend,” Nester said. “Overnight temperatures will be above freezing where we still have snow, which will help to accelerate the melt, at least at mid-slopes this weekend. Then we get a change, with a moist southwest flow pattern that could bring daily showers, maybe even thunderstorms, for the middle of next week.”

Flooding isn't anticipated, he said.

That should be good news for Orchard Homes residents whose properties flooded last year, when the Clark Fork River flows reached 7.5 feet and started flowing through their yards. The impacts at that level prompted officials to lower the minor flood stage to 7.5 feet this year.

Funding for the mapping project includes $15,000 from the Missoula Conservation District, which will use the mapping during permitting processes for those who are within the channel migration zone. Ross said he hopes to get another $5,000 from the Department of Justice’s Natural Resource Damage Program.

The Missoula Water Quality District will add about $20,000 more to the project. Ross said while they think the work only will cost $32,000, they’re hoping to put $40,000 into the pot in case they have unexpected costs.

“We’re building in a buffer so we can accommodate any changes that are unforeseen. Like right now, if we have to bump around the aerial photography until the fall, our costs would go up,” Ross said.

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